The applications and principles of Raman spectroscopy are well-known and thus will not be described here in detail. Briefly, Raman spectroscopy is an in-elastic light scattering technique that that uses the Raman-effect. In a typical Raman spectroscopy system, an excitation laser illuminates a substance (more specifically, a sample of a substance) containing various molecules that provide the Raman scattering signal which is then collected to permit the determination of the various concentrations of the chemical constituents in the substance, and even their temperature from their characteristic Raman scattering signature provided in the spectrogram. Common to all methods of Raman spectroscopy (non-Fourier transform methods) systems is a requirement for a device or means to disperse the scattered light radiation into constituent wavelengths for the purposes of producing a Raman spectrogram showing the individual spectrally resolved vibrational or pure-rotational line intensities of the scattering species under investigation. Additionally, all Raman spectroscopy systems require a means of removing or attenuating the incident laser excitation wavelength which is many orders of magnitude (typically >106) more intense than the scattered Raman signal.
In the field of Raman spectroscopy, numerous types of spectrometers and spectrographs have been developed to address the unique requirements of high signal throughput necessitated by the low Raman signal intensity, efficient rejection of the laser excitation wavelength, good spectral resolution, and good spatial resolution for multi-channel imaging spectrographs. Systems that have been used for these purposes include: conventional Czerny-Turner reflection spectrographs; spectrographs with combined dispersive/focusing elements such as curved gratings; axially-transmissive lens spectrograph designs; systems using holographic volume phase gratings; ion-beam etched curved gratings with blaze angles; multiple spectrographs in double-subtractive mode followed by a conventional dispersive mode, also known as a triple-spectrograph. Most of these systems have been developed for use in a laboratory environment, and as a result, can be quite fragile and sensitive to misalignment, they also lack the robustness, compactness, and cost-effective construction required for routine deployment in the field such as on-line industrial process automation and control. There are more rugged systems that were developed for use in the field such as the design described in U.S. Pat. No. 5,011,284 to Tedesco et al; Smith et al., U.S. Pat. No. 6,028,667 to Smith et al; U.S. Pat. No. 5,644,396 to Hopkins, and even larger systems such as the Holospec f/1.8i manufactured by Kaiser Optical Instruments, Ann Arbor, Mich. However, these prior-art designs do not incorporate design methodologies shown in the present design which incorporates features that will enable a simple, rugged, cost-effective and easy-to-use design, with a combined spectral filter assembly for the purposes of rejecting the elastic light scattering without the use of one or more additional lens assemblies.